Demand for wireless communication services, such as mobile telephone services in cellular and PCS systems, is continually increasing. An important issue in wireless communication systems involves the estimation of mobile station location. For example, the Federal Communications Commission (FCC) has requested that all cellular and PCS systems eventually include emergency 911 caller location capabilities similar to those provided in wired systems. As noted in Radio Communications Report, Vol. 15, No. 51, Dec. 16, 1996, the FCC has required that Phase I of a wireless emergency 911 (E-911) system providing a 911 agent with caller number and cell site location must be completed by Apr. 1, 1998, while Phase II of the E-911 system providing caller latitude and longitude within a radius of no more than 125 meters in at least 67% of all cases must be completed by Oct. 1, 2001. A number of other services requiring mobile location estimation are also being considered, including routing guidance service, fleet management and other commercial services. A wireless system which is able to determine the position of a given mobile station in an efficient manner could thus provide an enhanced level of service to the user, while meeting the above-noted FCC requirements and also generating additional revenue for the service provider.
Many conventional techniques for estimating mobile location in a wireless system are based on time difference of arrival (TDOA) measurements, which involves measuring the difference in arrival time of signals transmitted to or from different locations in the system. When implemented in the base-to-mobile direction, TDOA location estimation involves the mobile station detecting signals transmitted from at least three surrounding base stations. When implemented in the mobile-to-base direction, TDOA location estimation involves at least three surrounding base stations detecting a location signal transmitted from the mobile station. In either case, the resulting signal arrival time information can then be processed using well-known relationships to derive an estimate of mobile location. Three base stations are generally required in order to estimate mobile location in two dimensions. Differential range values may be computed by multiplying the TDOA differential path delay measurements by the speed of light c to provide an estimate of the differential distance between the mobile station and any pair of the three base stations. Each differential range defines a hyperbola having its foci at the corresponding base stations, such that the mobile location may be estimated as the intersection of three hyperbolas associated with the three pairs of base stations. A two-dimensional mobile station location estimate (x.sub.0, y.sub.0) may be generated by solving equations for the hyperbolas using differential range values computed for the first and second, first and third and second and third base stations. Additional details regarding these and other conventional location estimation techniques may be found in, for example, J. Caffery et al., "Radio Location in Urban CDMA Microcells," Proceedings of PIMRC '95, pp. 858-862, IEEE, 1995, and M. Wylie et al., "The Non-Line of Sight Problem in Mobile Location Estimation," ICUPC '95, Boston, Mass., 1995, both of which are incorporated by reference herein.
Regardless of the manner in which the differential range values are processed to determine mobile location, the TDOA location estimation process is often complicated by the limited bandwidth available for the time difference measurements. In general, a wider bandwidth provides a more accurate arrival time measurement for a given location signal. The multipath environment in which many wireless systems operate further complicates the measurements. The arrival time of interest for a given location signal is that corresponding to the most direct path between a transmitter and receiver. This arrival time will be delayed by an amount of time proportional to the distance or range between the transmitter and receiver, and is therefore useful in location estimation. The signals carried by other non-direct paths can be regarded as a form of noise. Another source of impairment is additive noise due to interference from other mobile stations or base stations. For example, when a mobile station is within about one-fourth of a cell radius of a given base station, a location signal transmitted from or received by that base station can be as much as 35 dB stronger than the corresponding signal associated with the third nearest base station. Moreover, in wireless systems based on the IS-95 standard, the weaker signals will generally occupy the same frequency band as the stronger signals. In the same band there will also be signals transmitted to or received from other mobile stations, further degrading the signal-to-interference ratio (SIR). Under these conditions, successful reception of the weaker location signals becomes increasingly difficult as the mobile gets closer to a given base station.
It is therefore important that a system with TDOA-based mobile location estimation provide sufficient bandwidth to discriminate a desired direct path location signal from the unwanted multipath signals, and sufficient signal-to-noise ratio (SNR) to enable the possibly very weak direct path signal to be detected in the presence of interference from other mobiles or base stations. Unfortunately, calculations indicate that the 1.25 MHz bandwidth typical of a given IS-95 communication channel is not adequate to achieve the desired TDOA accuracy. For example, a bandwidth of about 10 MHz would typically be required to provide a 100 ns resolution for the TDOA measurements. In addition, the fact that many IS-95 system base stations utilize the same frequency bands also makes the available SNR inadequate. Application of conventional TDOA-based mobile location estimation techniques to IS-95 and other CDMA wireless systems may therefore require alteration of basic system parameters, thereby increasing the cost and complexity of the system and possibly degrading system performance in terms of interference and voice quality.
As is apparent from the above, a need exists for a mobile location estimation technique which provides a broader bandwidth and enhanced SNR for transmitted location signals, without altering basic system operating parameters, without requiring substantial additional mobile station and base station circuitry, and without significantly degrading voice quality and other performance measures in the wireless system.